1. Field of the Invention
The present invention relates generally to a process for the separation of relatively less volatile hydrocarbon constituents from a volatile methane rich gas stream and, more specifically, to such a process in which ethane recovery is minimized due to required operating economies.
2. Description of the Prior Art
Ethane, propane and heavier hydrocarbon constituents can be recovered from a variety of gases, such as natural gas, refinery gas, and synthetic gas streams obtained from various sources. Natural gas ordinarily contains a major proportion of methane and ethane, these constituents comprising greater than fifty mole percent of the feed gas. The gas may also contain lesser amounts of heavier hydrocarbons such as propane, butanes, pentanes, and the like as well as hydrogen, nitrogen, carbon dioxide and other gases. A typical analysis of a feed gas processed in accordance with the present invention might contain, for example, in approximate mole percent, 92.5% methane, 4.2% ethane, and other C.sub.2 components, 1.3% propane, and other C.sub.3 components, 0.4% isobutane, 0.3% normal butane, 0.5% pentanes plus, with a balance of the feed gas comprising nitrogen and carbon dioxide. In some situations, sulfur containing gases are also present.
Recent substantial increases in the market for ethane and propane components of natural gas have provided a demand for processes which yield higher recovery levels of these products. The cryogenic expansion type recovery process is often preferred for ethane recovery because it is generally simpler than alternative schemes, providing ease of startup, operating flexibility, good efficiency and reliability.
In the typical cryogenic expansion recovery process, a feed gas stream under pressure is cooled by heat exchange with other streams of the process and/or external sources of refrigeration such as a propane compression-refrigeration system. As the gas is cooled, liquids are condensed and collected in one or more separators as high-pressure liquids containing some of the desired C.sub.2 +components. The high pressure liquids are expanded to a lower pressure and fractionated. The vaporization occurring during expansion of the liquid results in further cooling of the remaining portion of the liquid. The expanded stream, comprising a mixture of liquid and vapor, is fractionated in a distillation (demethanizer) column. In the column, the expansion cooled streams are distilled to separate residual methane, nitrogen, and other volatile gases as overhead vapor from the desired ethane, propane and heavier components as bottoms liquid product.
Typically, the feed gas is not totally condensed and vapor remaining from the partial condensation is passed through a turbo-expander, or expansion valve, to lower the pressure of the stream to a point at which additional liquids are condensed as a result of further cooling of the stream. The pressure after expansion is approximately the same as the pressure at which the distillation column is operated. Liquids thus obtained are supplied as feed to the column. Although condensed liquid is typically expanded in these processes through, e.g., a valve, to column pressure, the primary expansion involved is gas expansion with work recovery and resulting cooling of the gas, which forms additional liquid. Usually, the remaining vapor and the column overhead vapors are combined as a residue methane rich product gas.
In the typical process for recovery of ethane and heavier hydrocarbon constituents from a hydrocarbon gas stream, the residue gas leaving the process will contain substantially all of the methane in the feed gas with essentially none of the heavier hydrocarbon components. The bottoms fraction from the demethanizer column will contain substantially all of the heavier components with essentially no methane or lighter components. At times, fluctuations in the prices of both natural gas and its NGL constituents reduce the incremental value of ethane and heavier components as liquid products. On these occasions, operating economies require that ethane recovery be minimized. This alternative operation is typically accomplished by raising the demethanizer bottoms temperature and rerouting side reboiler connections to the demethanizer, along with reheating feed streams to the demethanizer. Many piping changes are required to accomplish ethane rejection according to conventional practice. Also, ethane rejection can require a larger diameter tower to accommodate the rejected ethane vapor.
It is an object of the present invention to provide a process for ethane rejection in a cryogenic recovery process in which stripping gases present in the inlet gas are used to strip out ethane at a lower temperature than usually required with few, if any, piping realignments being required.
Another object of the invention is to provide an ethane rejection process utilizing a stripping gas in a fractionating column in which there is no need for increased column diameter.
Another object of the invention is to accomplish ethane rejection with ease of operation, lower capital investment requirements, and with increased simplicity of piping, lower reboiler duties, and a smaller required column diameter.